Presently, sight defects such as myopia (short-sightedness), hyperopia (far-sightedness or long-sightedness) or astigmatism can be corrected permanently by means of refractive-surgical treatment. Refractive-surgical treatments are surgical procedures on the eye which change the optical refractive power of the eye with the aim of approximating it as closely as possible to a desired value. These days, laser technology is typically used for treating eye tissue, e.g. for tissue cuts and tissue reduction. In particular, highly focused femtosecond laser pulses with pulse widths of typically 100 fs to 1000 fs (1 fs=10−15 s) are used for the tissue treatment. The cornea of the eye is currently treated using commercial femtosecond laser systems. However, since other tissue parts and tissue regions of the eye, such as the sclera, lens and retina, are also treated with the laser pulses, different working distances and hence focal distances are required for the different areas of application; this requires not only mechanical positioning mechanisms but also different light projection optics. Additionally, different applications and treatment regions often also require image fields (i.e. image regions in focus) with differing size and/or curvature, which in turn can be achieved by means of corresponding application-specific projection optics. In order to obtain different, application-specific focal diameters, focal shapes and focal extents in the projection direction and/or beam divergences, different projection optics with respectively different numerical aperture (NA) must likewise be used. Finally, different projection optics, in particular transparent application elements such as contact bodies, e.g. applanation bodies or distance bodies, may be required due to the specific optical properties of patient interfaces to be implemented. Flexible adaptation of the projection optics to the different application-specific demands could in an ideal case be made possible by a suitable varifocal lens.
Generally, a high NA is desirable because high NA allows the generation of small focal points (spot size), and hence a smaller cut-zone per pulse. It is actually very difficult to produce varifocal lenses which cover a large work region as a result of the often desired high NA, short working distances and in general very small focal sizes (spot sizes). If, additionally, a weight which is as low as possible and a small overall size are decisive (this can easily be the case for use on the eye with a small working distance), then the construction of suitable varifocal lenses is made even more difficult. Image fields with different curvature cannot be implemented by varifocal lenses known from the prior art. Since, for example, the retina and the cornea have a different curvature, different image field curvatures are also necessary.